Nerve cells of vertebrata need a certain group of polypeptides, called a neurotrophic factor, for their survival. As one of the neurotrophic factors, a brain derived neurotrophic factor (BDNF) has been known. BDNF has first been isolated from porcine brain by Barde, Y. E. et al. (cf., The EMBO Journal, 5, 549-553 (1982)), and thereafter BDNF genes of pig, human and mouse have been cloned in 1989 whereby it has been confirmed that it has a primary structure consisting of 119 amino acids (cf., Leibrock, J. et al., Nature, 341, 149 (1989)). Recently, a lot of attention has been given to BDNF because BDNF is considered to play a very important role in the central nervous system.
BDNF is a polypeptide exhibiting various pharmacological activities in the nervous system, and the pharmacological activities thereof have been disclosed in Seitai no Kagaku (Science of Living body), 43 (6), 616-625 (1992). Besides, BDNF has been expected to be useful as an agent for treatment of various diseases such as amyotrophic lateral sclerosis (ALS), anticancer agent-intoxicated neuropathy, diabetic neuropathy, retinal pigment degeneration, glaucoma, Huntington's chorea, Parkinson disease, Alzheimer's disease, terminal cancer ache, depression, obesity, etc., based on the pharmacological activities thereof (cf., U.S. Pat. No. 5,180,820, Seitai no Kagaku, 43 (6), pages 616-625, (1992)).
A compound to be used as a medicament is usually required to be stable without a change in the activity thereof over time under conventional storage conditions when formulated in a form of a conventional pharmaceutical composition. Especially, highly purified polypeptides such as BDNF have many problems to be solved in order to be kept stably for a long time. For example, when keeping BDNF in the form of a solution in a conventional physiological saline solution, BDNF has a tendency to aggregate even where stored for several days to dozens of days, which is a very serious problem. The aggregate of BDNF is known to cause immune toxicity to the living body, so that the prevention of the production of BDNF aggregate is very important. Besides, denatured and/or polymerized BDNF are often produced as well. Hitherto, there has been no report on effective ways to solve these serious problems of BDNF.
In general, when a compound of a low molecular weight is unstable in the form of an aqueous solution, the solution is usually tried to be lyophilized for stabilization. However, polypeplides are known not to be stable during the lyophilization procedures (cf., Tanpakushitsu, Kakusan, Koso (i.e., Protein, Nucleic Acid, Enzyme), 37 (9), 1517 (1992)). Besides, the conventional stabilizers exhibit their stabilization effects in an aqueous solution of a polypeptide by supporting hydration between a water molecule and a polypeptide. Therefore, these stabilizers cannot exhibit their stabilization effects in a lyophilized composition in many cases because no water molecule exists therein (cf., Tanpakushitsu, Kakusan, Koso (i.e., Protein, Nucleic Acid, Enzyme), 37 (9), 1517 (1992)). Hitherto, a lyophilized pharmaceutical composition of BDNF has been known, and any skilled person in the art cannot imagine the physicochemical and biological stability of the lyophilized pharmaceutical composition of BDNF.